I would solve this using unfoldr from Data.List. f :: Maybe (String, String, String, [String]) -> Maybe (String, String) f = fmap (\(_,_, rest, xs) -> (concat xs, rest)) -- *Main> unfoldr (f . matchRegexAll re) str -- ["fish1","cow3","boat4"] HTH Thomas Am 15.12.2014 um 06:18 schrieb Cody Goodman:

I've had some trouble wrapping my brain around how to use a scan to build this list:

code below, but here's an lpaste link: http://lpaste.net/116494

{-# LANGUAGE OverloadedStrings #-} import Control.Monad import Data.Maybe import Text.Regex

type RegexRes = Maybe (String, String, String, [String])

re = mkRegex "\\((\\w+)\\):([[:digit:]]+)"

getParenNum :: String -> [String] getParenNum s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num):getParenNum after

getParenNumOnce' :: String -> String getParenNumOnce' s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num)

-- trying to accumulate the results of regexMatching repeatedly over a string f1 str = (\ (_, _, xs, acc) -> Just (xs, acc)) <=< (matchRegexAll re) $ str

f2 str = fromMaybe "" $ (\ (_, _, after, [word,num]) -> Just after) <=< (matchRegexAll re) $ str

-- f3 :: RegexRes -> RegexRes -- f3 :: RegexRes -> String f3 match = fromMaybe [] $ (\(_,_,after,[word,num]) -> Just (word ++ num)) =<< match

f4 str = scanl (const f3) "" [(matchRegexAll re str)]

f5 acc str = fromMaybe [] $ (\ (_, _, after, [word,num]) -> Just (acc ++ [word ++ num])) <=< (matchRegexAll re) $ str -- λ> scanl f5 [] ["(fish):1 sausage (cow):3 tree (boat):4"] -- [[],["fish1"]]

str = "(fish):1 sausage (cow):3 tree (boat):4"

-- λ> foldl (const . f3 . (matchRegexAll re)) "(fish):1 sausage (cow):3 tree (boat):4" ["(fish):1 sausage (cow):3 tree (boat):4"] -- "fish1" -- λ> -- want ["fish1","cow3","boat4"] -- λ> -- I feel like I should be using something in place of const that smartly chooses whether to pass String or... idk

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